TERTIARY VOLCANIC ROCKS-LATITE-PHONOLITE. 
69 
characterized by phenocrysts of soda-lime feldspar and ferromagnesian silicates 
in a groundmass rich in potash. 
Latite-phonolite differs from phonolite in having lower alumina and alkalies 
and more iron, magnesia, and lime. It differs from latite in having somewhat 
lower silica, iron, magnesia, and lime, and higher alumina and alkalies, with soda 
predominant over potash. It is intermediate between latite and phonolite, chemi¬ 
cally, in having each essential oxide present in an amount which lies between the 
average present in latite and that in phonolite; mineralogically, in that the amount 
of minerals high in alumina and soda—such as nepheline—of the phonolites is 
smaller in the latite-phonolites, that part of the alkali feldspar of the phonolites 
unites with the lime-rich feldspars of the latites to give a rather alkalic plagioclase, 
and that the dark silicates are intermediate in composition between those of phono¬ 
lite and those of latite. The group thus broadly characterized shows, as is not 
uncommon in high alkali rocks, a notable prominence of lime over magnesia. 
Such a composition, of course, allows the development of orthoclase and albite 
molecules, and the considerable lime, low magnesia, and plentiful alumina necessi¬ 
tate the formation of a comparatively small amount of the anorthite molecule. 
Both orthoclase and sodic plagioclase are thus characteristic minerals of this group. 
For true phonolite nepheline is considered one of the essential components, but in 
this group, at least, it seems desirable to make the distinctions chemical rather than 
mineralogical by giving to each of the minerals analcite, sodalite, and nosean a 
classificatory rank equal with nepheline. The last two minerals have practically 
the same ratios of silica, alumina, and soda as nepheline, and in analcite only the 
silica is a little different; it is probably only because of the presence in the magma 
of water, chlorine, or sulphuric anhydride, respectively, that these minerals form 
instead of nepheline. Pyroxene or amphibole are common constituents. The) T 
are often alkalic, always low in magnesia, and contain a large percentage of the 
total lime ol the rock. Mica, if present, must, on account of the low magnesia, be 
an iron-rich variety, like lepidomelane. Apatite and sphene are often noteworthy 
accessories and help to utilize the lime. 
The characteristic minerals of the latite-phonolites are therefore orthoclase, 
sodic plagioclase, a soda feldspathoid (including analcite), and some mineral of the 
pyroxene, amphibole, or mica groups. The proportions of essential oxides consid¬ 
ered to represent a typical latite-phonolite are here given: 
Typical composition of latite-phonolite. 
SiOo. 55.00 MgO . 1.00 
A1 2 0 3 . 19. 00 CaO . 4. 00 
FeO J. 50 | K 2 0. 4.50 
These rocks show affinities to several surrounding types besides phonolite and 
latite. The relation to trachyte is in many ways very close. In fact, the term 
trachyte-phonolite is almost as correctly applicable to these rocks as latite-phonolite. 
Silica is lower than in trachyte and alumina and ferric iron a little higher, while lime 
is noticeably more abundant. The amount of alkalies is slightly greater, but the 
ratio of soda to potash is inverted. These chemical differences from trachyte 
